1. Field of Invention
This invention relates to an improvement in a process for reducing draw-resonance in polymeric film.
2. Definitions
By draw-down is meant the stretching of an extruded web or film in the direction of flow and sometimes also in the transverse direction.
By "draw ratio" is meant the film or coating velocity at the rapid cooling zone or station divided by the average extrudate velocity emerging from a die. This can be approximated by using the inverse of thicknesses which can be expressed as ##EQU1## wherein t.sub.f is the thickness of film or coating;
t.sub.o is the thickness of film extrudate at the die exit PA1 R is the draw ratio
By "draw resonance" or surging is meant periodic variations in the molten film occurring between the die and the rapid cooling zone. It occurs when the draw ratio exceeds a critical value. Draw resonance or surging can be thought of as an instability phenomenon manifested during the drawing down of a material coming from an extruder, more or less homogeneously. The instability manifests itself in the occurrence of periodic fluctuations in the extruded web (film) dimensions such as film thickness when a critical value of the draw ratio is exceeded. Surging may be so extreme as to actually break a web or film that extrudes from a die and totally shuts down an extrusion coating or film process.
3. Description of the Prior Art
Certain polymeric resins and particularly polyolefin resins are not entirely satisfactory for commercial operations such as melt embossing and/or extrusion coating applications. This has been primarily attributed to the phenomenon of draw resonance.
Normally, the onset of draw resonance with certain polymeric resins requires two fixed points; an extrusion die and a rapid cooling zone and both are present in extrusion coating and embossing processes. Draw resonance for certain resins occurs at characteristic draw ratios, often as low as 5 to 1. In both melt embossing and extrusion coating applications, draw resonance can occur with certain resins because die gaps are usually 20-40 mils and the thickness of the drawn down final film or coating is usually equal to or less than 1 mil. Thus, the draw ratio may be in excess of the critical draw ratio of certain resins.
One possible method for reducing draw resonance is to lower the draw ratio which is approximately equal to the ratio of the die gap to the film or coating thickness. However, at the high takeoff speeds (equal to or greater than 500 FPM) and thin gauges (equal to or less than 1 mil) encountered in these processes, it is not commercially feasible to reduce the die gap, thereby lowering the draw ratio, because of constraints of head pressure and melt fracture. It has presently been proposed to overcome draw resonance with difficult processing polymeric materials particularly polyolefins such as linear low density polyolefins (LLDPE), high density polyolefins (HDPE) and polypropylene by using mixtures of these polymeric materials with easier processing resins such as disclosed in U.S. Pat. No. 4,339,507. Unfortunately, however, in addition to higher raw material costs, the final product does not have the inherent property advantages achieved by using the LLDPE, HDPE, or polypropylene alone.
Another method for reducing draw resonance is disclosed in U.S. Pat. No. 4,486,377. Briefly, there is disclosed a process for reducing draw resonance of a polymeric film wherein a fluid medium is directed against the molten film substantially midway between the die and a rapid cooling zone.
Although this process is extremely effective in reducing draw resonance, there are however few instances where the film exhibits slight traces of draw resonance due to the difficulty of manufacturing and directing the fluid medium.